Liquid metals, such as mercury, have been used in switches to provide an electrical path between two conductors. An example is a mercury thermostat switch, in which a bimetal strip coil reacts to temperature and alters the angle of an elongated cavity containing mercury. The mercury in the cavity forms a single droplet due to high surface tension. Gravity moves the mercury droplet to the end containing electrical contacts or to the other end, depending upon the angle of the cavity. In a manual switch, a permanent magnet is used to move a mercury droplet in a cavity and bring it into contact with electrical contacts.
Liquid metal is also used in relays. A liquid metal droplet can be moved by a variety of techniques, including electrostatic and electromagnetic forces, variable geometry due to thermal expansion/contraction, and magneto-hydrodynamic forces.
Rapid switching of high currents is used in a large variety of devices, but provides a problem for solid contact based relays because of arcing when current flow is disrupted. The arcing causes damage to the contacts and degrades their conductivity due to pitting of the electrode surfaces. Liquid metal switches can overcome this problem.
Micro-electromechanical (MEM) systems also utilize liquid metal switching. When the dimension of interest shrinks, the surface tension of the liquid metal becomes dominant force over other forces, such as body forces (inertia). Latching switches are described in the co-pending patent applications that use liquid metal as the part that causes the electrical or optical signal to be routed in one path, blocked or routed in another path. Sometimes the use characteristic of these switches requires them to be actuated with higher than normal switching energies to move the liquid metal from one location to another after the switch-state has remained unchanged for several minutes. This phenomenon is called “kick-starting”. Kick starting is undesirable because it requires the use of at least two different switch drive energies, as well as keeping track of the time since the switch-state was last changed. Alternatively, repeated attempts to change the state of the switch must be made until a signal is received that the switch-state has been changed successfully. Both of these processes add complexity and waste switching time.